Tailoring Co-Loaded SnO ₂ Nanostructures as Next-Generation Photocatalysts for Efficient Hydrogen Evolution

The growing reliance on conventional chemical synthesis and the increasing global energy demand the necessity of sustainable energy technologies, in which green synthesis of materials and hydrogen production through photocatalytic water splitting can be a promising solution. This study reports the green synthesis of undoped and Co doped SnO2 nanostructures using Lepidagathis ananthapuramensis leaf extract as a natural reducing and stabilizing agent. This work systematically investigates the maximum loading of Co into the SnO2 lattice and its prospective application for photocatalytic hydrogen production. Structural characterization revealed a pure tetragonal phase of SnO2 persisted up to 7% Co substitution, with a secondary spinel Co3O4 phase appearing at 10% Co substitution, asserting that the maximum uptake of Co into SnO2 lattice is 7%. The average crystallite and particle size varied, which is influenced by Co content as verified by X-ray diffraction and transmission electron microscopy techniques. The optical characterization revealed the systematic narrowing of band gap from 3.10 to 2.29 eV with an increase in the Co content ≤7%, enhancing visible light absorption due to oxygen vacancies and midgap states induced by Co substitution. Surface N2 adsorption analysis revealed an increase in the surface area for SnCo-7 (18.58 m2 g–1) as compared to SnO2 (7.80 m2 g–1), attributed to improved pore connectivity and distribution. The photocatalytic hydrogen production under visible light illumination increased significantly with an increase in the Co substitution, peaking at 1254.15 μmol g–1 h–1 for SnCo-7, approximately five times higher than that of the pristine SnO2. However, a higher Co doping level (10%) in SnO2 led to performance declines due to the formation of secondary phase Co3O4 and increased recombination of charge carriers. Our findings suggest valuable insight into the fabrication and optimization of single-metal Co doped SnO2 photocatalyst without making any nanocomposites or blending with carbon-based materials, marking its cost-effective potential for efficient and sustainable hydrogen evolution.